The adsorption of Hg on the gehlenite surface was investigated with density functional theory (DFT). The gehlenite structure was modeled with a periodic slab and was optimized with the local density approximation (LDA) with the Perdew – Wang (PWC) functional. The adsorption energies of Hg adsorbed on the gehlenite surface was calculated with the gradient corrected (GGA) method with the Becke-Lee-Yang-Parr (BLYP) functional using the lower level optimized structure. The fine basis set, core treatment and spin unrestricted methods were applied to all calculations to lead higher level and hopefully more accurate results. The equilibrium constants with respect to the temperature effects were also predicted, the results from this work show how Hg adsorbs on an atop site of oxygen with a shorter distance and stronger adsorption energy on gehlenite compared to other adsorbates.